How long does a cot bed last?

[cocopops]

We've got a Mamas and Papas Hampton cot bed.DD isn't in it yet (she's 2 in December) but wondered what age these generally last up to?

Supposed to 5 or 6 I think but one of my DDs is tall for age and had really grown out of it by her 4th birthday.

Funnily enough my DD has only just turned 4 & small for her age but I've noticed recently her feet aren't that far from the bottom. We've a Design Company cot-bed but we bought a M&P mattress so it's presuambly the same size. I thought it'd do her till she was about 6 or so especially as she's so small but it doesn't look like it will.